This invention relates generally to computed tomographic (CT) imaging, and more particularly to methods and apparatus for boosting image resolution for relatively long duration scans.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In known CT systems, multiple-sec scans are used to increase signal strength in certain calibration and patient scans, and to increase angular sampling. For example, one known scanner offers 2-sec, 3-sec, and 4-sec scan speeds in addition to the more commonly used 0.8-sec and 1-sec scan speeds. For a 2-sec scan speed, a gantry of the scanner rotates 360 degrees in 2 seconds, collecting 984xc3x972=1968 angular views. The view-to-view angular interval is 0. 18 degree. Views are combined before reconstruction to increase reconstruction speed Combining the views reduces angular sampling, which is disadvantageous in some cases.
If a view uncompressed reconstruction were used, the reconstruction kernel could be boosted to increase image resolution without increasing aliasing artifacts. Increased resolution without increased aliasing would be advantageous for imaging the head and spines. However, because of the slower rotation speed in multiple-sec scans, multiple-sec scans produce more focal spot movement than 0.8 and 1-sec scans. The performance on axial encoding accuracy is also better with 0.8 and 1-sec scans than with 2- and 4-sec scans.
It would therefore be desirable to provide methods and apparatus to provide multiple-sec scans suitable for use with view uncompressed reconstruction.
There is therefore provided, in one embodiment of the present invention, a method for collecting views for imaging an object utilizing a CT imaging system. The method includes steps of: performing a first rotational scan of an object to acquire a first set of views of the object; performing a second rotational scan of the object to acquire a second set of views of the object; offsetting starting angular views of the second rotational scan relative to the first rotational scan so th at the first set of views and the second set of views form a set of interlaced views; and reconstructing an image of the object utilizing the first set of views and the second set of views.
The above described embodiment provides shorter interlaced scans that reduce focal spot movement and encoding accuracy problems associated with slower scan speeds, while still providing increased image resolution by using kernel boosting without increasing aliasing artifacts.